Light-emitting diodes (LEDs) belong to a class of the most efficient light sources among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL) for backlight, and halogen lamps. As one configuration for white LED, a pseudo-white LED device constructed by combining a blue light-emitting diode with a phosphor capable of emitting light of longer wavelength, for example, yellow or green light upon blue light excitation is implemented on a commercial basis.
Known examples of the yellow phosphor which is combined with the blue LED to produce pseudo-white light include Y3Al5O12:Ce, (Y,Gd)3(Al,Ga)5O12:Ce, (Y,Gd)3Al5O12:Ce, Tb3Al5O12:Ce, CaGa2S4:Eu, (Sr,Ca,Ba)2SiO4:Eu, and Ca-α-SiAlON:Eu.
The mainstream of the white LED structure is a system in which a phosphor is placed on or near a blue LED so that the phosphor may convert the wavelength of part of blue light to produce white light. Also some devices are based on a system in which a phosphor is spaced apart from a blue LED by a distance of several millimeters to several tens of millimeters so that the phosphor may cause wavelength conversion to part of blue light. Particularly when the phosphor tends to degrade its properties by the heat generated by the blue LED, the far distance of phosphor from the blue LED is effective for preventing the phosphor from changing its color tone by the heat generation of the blue LED and suppressing a reduction of luminous intensity. The phosphor-containing wavelength conversion member used in this system is known as “remote phosphor” plate.
The known methods of disposing the phosphor as a wavelength conversion member at a spacing of several millimeters to several tens of millimeters from the blue LED include a method of mixing and dispersing the phosphor in a silicone resin or epoxy resin, and coating the resin to a transparent substrate, and a method of mixing, kneading and dispersing the phosphor in a thermoplastic resin, molding the resin, and placing the molded part at a spacing from the blue LED. Of these, the method of mixing, kneading and dispersing the phosphor in a thermoplastic resin, molding the resin, and placing the molded part as a wavelength conversion member is employed in most cases because of freedom of choice of the strength and arrangement of the resin layer.
Notably, the preceding documents pertinent to the present invention include, for example, U.S. Pat. No. 6,350,041 (Patent Document 1), JP-A 2012-153904 (Patent Document 2), and JP-A 2012-017454 (Patent Document 3).